Customarily, to manufacture a metal tube continuously and on an industrial scale, use is made of a metal sheet or strip, the width and thickness of which are suited to the tube which is to be produced, that is to say are compatible with the end-use of the tube.
To do this, the metal sheet or strip is placed on a device that allows the metal sheet to be given a non-zero rate of travel along its longitudinal axis. As it travels, a succession of rollers gives this strip, first of all, a substantially U-shape, then a substantially O-shape, by bringing the two longitudinal edges of the strip closer together, the two longitudinal edges being substantially straight and parallel to one another.
When the tube has to contain fillers such as powders, granules or mixtures thereof, these are introduced into the tube when it is in the U-shape.
These fillers are introduced by means of a belt on which the fillers are placed and which runs in synchrony with the progression of the strip, so that the fillers are continuously tipped from the belt into the channel that the U-tube forms. The amount of fillers tipped out by the belt is altered by adjusting the height of the fillers on the belt.
Next, as before, a new series of rollers allows the tube to be given its O-shape, that is to say its substantially cylindrical shape.
The two longitudinal edges of the strip, which find themselves facing each other, are generally joined by welding the said edges together.
At the present time, at least two welding methods are used industrially, namely the high-frequency welding method and the multi-cathode TIG method.
However, these each have a certain number of drawbacks.
Thus, when use is made of a high-frequency welding method for welding the longitudinal edges of a tube containing powders that are not completely non-magnetic, these powders are "drawn" by the effect of the very intense magnetic field created by the high-frequency welding current and then contaminate the weld, causing defects which lead to greater weakness of the welded joint which can then no longer withstand without rupturing later transformations, such as subsequent drawing and/or rolling stages.
Documents EP-A-0158691, EP-A-0158693, EP-A-0589470, U.S. Pat. No. 4,584,169, U.S. Pat. No. 4,632,882 and U.S. Pat. No. 5,192,016 have already emphasized this problem and attempted to provide solutions. However, none of these solutions is truly satisfactory from the industrial point of view because these solutions impose, in particular, tight constraints on the filler powders to be used or require very expensive and not very profitable investment.
An alternative lies in the multi-cathode TIG welding method, which is used for manufacturing tubes made of non-ferromagnetic material, such as austenitic stainless steels or copper-containing alloys.
In general, this technique employs three electrodes aligned in the direction of travel of the tube and distant from one another by a constant value which should ideally be reduced to the minimum, but which depends on the size of the various constituent members, on requirements for electrical insulation of the electrodes and on requirements for cooling the entire assembly.
Welding rates for multi-cathode TIG welding depend, in particular, on the thickness of the welded joint and on the nature of the material, therefore on the thermal diffusivity. However, for similar geometries, when both methods can be used, high-frequency welding can be carried out at far higher speeds than multi-cathode TIG welding.
This is because, in high-frequency welding, the energy is generated across the entire thickness of the edges to be welded together, whereas in multi-cathode TIG welding, the energy of the arc is transmitted only to the surface of the edges and then has to diffuse through the thickness, so that the lower face can be brought up to a temperature higher than the melting point of the material in question.
Put another way, the multi-cathode TIG method leads to far lower productivity than high-frequency welding, thereby penalizing its industrial benefit.
Furthermore, the multi-cathode TIG method presents a major drawback, namely the phenomenon of "magnetic blow", which is manifested by uncontrolled interactions between the electric arcs, which interactions force a limitation of the strengths of current employed in each of the electrodes and therefore, as end-result, force a considerable reduction in welding rates.
Now, weld penetration, that is to say the thickness of strip that can fuse in order to weld the two edges together, depends in particular on the strength of the current used in the multi-cathode TIG method and on the rate of welding.
All other things being equal, weld penetration is a function of the welding energy (E), which can be expressed by formula I below: ##EQU1## in which: Ui is the welding voltage (in volts) of cathode i;
Ii is the strength (in amps) of the current passing through the cathode i; PA1 Vs is the rate of welding (in cm.s.sup.-1); PA1 and n is the number (n.gtoreq.1) of cathode(s) i used. PA1 the contamination of the molten metal with the filler leads to the formation of defects or, at the very least, to the formation of a hard alloy which is weak and cannot be deformed without causing cracks, which cracks act as initiators for later rupture of the tube; PA1 the partial melting of the fillers inside the tube gives rise, after solidification, to large undeformable particles which damage the tube and may go so far as to cause it to rupture when, during a later rolling or drawing operation, the inside diameter of the tube becomes of the order of magnitude of the size of the particles resulting from the melting; and PA1 the volatilization of filler inside the tube causes some of the molten weld to be expelled outwards because of the increase in internal pressure in the tube. PA1 the electrode-plane/ground lead-plane distance (ei) is between 0 mm and +20 mm and preferably between +7 mm and +15 mm; PA1 use is made of at least two electrodes (Ei) and of at least two corresponding ground leads (PMi) and, preferably, of at least three electrodes (Ei) and of at least three corresponding ground leads (PMi); PA1 the separations between the electrodes are adjusted in order to ensure that the foot of each electric arc attaches to solidified metal; PA1 the separation between successive (E.sub.1) and (E.sub.2) electrodes is less than or equal to the separation between successive (E2) and (E.sub.3) electrodes; PA1 the intensity of the current passing through an electrode is greater than or equal to 50 A, and preferably greater than or equal to 100 A; PA1 the relative speed of travel of the sheet is greater than or equal to 2 m.s.sup.-1, preferably greater than or equal to 5 m.s.sup.-1 ; PA1 it further comprises a stage of chamfering at least part of the two longitudinal edges of the metal sheet, so that it presents, prior to welding, a V-shaped groove with a V-angle (.alpha.) of between about 30.degree. and about 120.degree., preferably between about 60.degree. and about 90.degree.; PA1 the groove has a height of between about 1/3 and about 2/3 of the thickness of the metal sheet; PA1 the metal sheet is filled with at least one filler prior to welding; PA1 the two longitudinal edges of the metal sheet are welded together incompletely by partial fusion of about 20% to about 80% of the thickness of the sheet at the edges and, preferably, of about 40% to about 60%; PA1 the edges are also forge-welded together by applying a lateral mechanical pressure to at least part of the outside wall of the sheet, so as to obtain a substantially complete weld throughout the thickness; PA1 after welding the tube undergoes at least one stage of drawing, rolling and/or recrystallization annealing.
The welding energy E is then expressed in J.cm.sup.-1.
From this it will be understood that if the rate of welding is to be increased while maintaining the same penetration, it is necessary to increase the welding energy, that is to say the power delivered by the generator or generators by increasing the current and/or the voltage on one or more of the cathodes.
However, in practice, it is difficult to gain a significant advantage from increasing the voltage because in a multi-cathode TIG welding method, the voltage is an increasing function of the arc height.
Now, a tall arc is not favourable to high-speed welding. This is because a tall arc leads to a reduction in the power density of the arc at the surface of the workpiece, and this has the effect, all other things being equal, of broadening the bead of welding and therefore decreasing the penetration, something which goes against the desired objective.
Furthermore, a longer arc is, on the one hand, more sensitive to magnetic interactions generating a deflection of this arc and, on the other hand, tends to attach more readily and for longer to the hot spot, that is to say to the molten metal, and this first of all results in a lengthening and bending of the arc, then leads to sudden detachment of the arc when the anode spot, that is to say the root of the arc, repositions itself vertically in line with the cathode. This then yields a discontinuous weld characteristic of an excessively high welding rate.
The result of this is that the increase in power of the electric arc, needed to maintain penetration, can be the result only of an increase in current.
Now, when the current is significantly increased, on one or more of the cathodes employed, there is observed increased instability of the electric arcs and the formation of a weld bead which is very "wavy" that is to say comprises a series of craters and lumps on the surface of the weld bead.
This phenomenon is known as "magnetic blow".
It will be readily understood that such a "wavy" weld bead is not able to allow subsequent rolling and/or drawing that is free of problems of rupturing.
Furthermore, other problems arise when the tube to be welded has been filled with fillers, for example metal powder.
In this case, carrying out full-penetration welding using a multi-cathode TIG method causes, somewhat randomly, partial fusion of the filler it contains and/or contamination of the molten metal which should normally be the result only of the melting of the strip.
It then follows that the tube can no longer be rolled and/or drawn without this operation causing rupture, given that:
Document U.S. Pat. No. 4,396,820 proposes a first solution to combat this phenomenon, namely that of lancing one of the edges to be assembled so as to allow the tube to be welded throughout its thickness without it being possible for the molten metal to come into contact with the fillers contained in the tube.
However, this method has the drawback of complicating the forming operation, of reducing the "free" volume inside the tube and of adversely affecting the axial symmetry of the tube.
Another solution might consist in performing welds with partial penetration, that is to say in causing only part of the thickness of the strip to melt at the two longitudinal edges which are to be welded together. However, welds with partial penetration are unanimously advised against by those skilled in the art because that part of the plane of the joint not welded constitutes a notch under the root of the weld bead, which is reputed to propagate during any reduction in section of the tube imposed by a subsequent drawing or rolling stage.